SEPARATION OF LIQUIDS

Abstract: A method of separating a first liquid from a second liquid in a mixture or suspension of the two liquids, the first liquid having a substantially higher electrical conductivity than the second liquid, which method includes the step of applying a high frequency electromagnetic field through the mixture or suspension.

Description: SEPARATION OF LIQUIDS The present invention relates to the separation of liquids and in particular to the improved operation of certain solvent extraction systems which include liquid/liquid contactors and liquid separators. According to the present invention there is provided a method of separating a first liquid from a second liquid in a mixture or suspension of the two liquids, the first liquid having a substantially higher electrical conductivity than the second liquid, which method includes the step of applying a high frequency electromagnetic field through the mixture or suspension. The said field may be a single phase or multi-phase field. The first liquid may be a relatively good conductor and the second liquid may be a relatively poor conductor, eg an insulator. For example, the first liquid may be water or an aqueous solution. The second liquid may be an organic liquid such as an organic solvent, fuel, oil, grease or the like. In the method of separation in accordance with the present invention the step of applying an electromagnetic field may be used in addition to other steps, eg the effect of gravity, to separate the liquids. The principle of the invention is that the applied field induces eddy currents in the first liquid in a manner similar to the MHD (magneto hydrodynamic) effect which thereby becomes a current carrying conductor and experiences a force which assists the separation process. Desirably, the applied electromagnetic field has a frequency greater than 5 kHz and more preferably greater than 10 kHz. The applied frequency may for example be from 50 kHz to 500 kHz. Desirably, the applied field is sufficiently great to generate a force per unit volume greater than the density difference between the first and second liquids but not sufficiently great to boil either of the liquids by the heat generated therein. In a first example of the method according to the present invention the separation process may be part of a solvent extraction mixer settler process in which a first liquid comprising a first solvent, eg an aqueous solution such as nitric acid, is contacted with a second liquid comprising a second solvent, eg an organic solvent, whereby chemical species dissolved in the first liquid and to be removed therefrom are extracted by preferentially dissolving in the second liquid. For example, metal species such as actinides to be recovered from reprocessed nuclear fuel in an aqueous solution, eg mineral acid solution such as nitric acid, are extracted by an organic solvent, eg dibutoxy diethyl ether. The two liquids are thereafter separated by allowing settling to take place at a differential rate by the effect of gravity whereby the first and second liquids separate into two layers. It is not possible to separate aqueous solutions and solvents which have the same density by the known method. However, by using an applied electromagnetic field in accordance with the present invention such liquids may be separated. Where the densities of the two liquids are different the applied field may be used in conjunction with the effect of gravity to separate the liquids thereby reducing the residence time of the liquids in the settler vessel employed to separate them. In a second example of the method according to the present invention, the method may be one employed to separate water or a watery substance from a fuel oil containing the same. For example, water droplets contained in aviation spirit may be separated therefrom. The separation may be carried out in a pipe or vessel or other suitable containment for the two liquids. Where the separation is carried out in a pipe along which the oil/water suspension is travelling the applied field may be sufficient to hinder or halt the progress of the water droplets which may be collected and extracted in the vicinity of the applied field. In a further example of the present invention the method may comprise an improvement to a solvent extraction process employing a pulsed column. In a known process employing such columns an aqueous solution and an organic liquid are contacted together in a column comprising a series of perforated plates. Droplets of the aqueous solution are formed by the perforations and fall through the column and contact organic solvent rising up the column. A mechanical or pneumatic pulse is conventionally applied to the rising organic liquid causing the mixed phase to oscillate through the perforations giving suitable aqeuous phase droplet formation and hence suitable mass transfer between the first and second liquids. Chemical species, eg metal ions, dissolved in one liquid are transferred to the other liquid which is thereafter allowed to separate. The application of an electromagnetic field in accordance with the present invention causes the aqueous liquid to oscillate by the force on the liquid thereby to assist the mass transfer process with or without a conventional applied mechanical or pneumatic pulse of energy. The applied field may be arranged to be applied in localised regions in the column, eg adjacent to regions close to one or more of the perforated plates and this thereby avoids the need to oscillate the whole liquid body as in the prior art. The applied field may alternatively be applied along substantially the entire length of the column. The applied field is desirably applied in pulses, eg having a pulse frequency of 0.1 to 10 Hz and a mark/space ratio of from 1/10 to 3/1. In another known process employing such columns an aqueous solution and an organic liquid are contacted together in a column comprising a series of perforated plates. Droplets of the organic solution are formed by the perforations and rise through the column and contact aqueous solution falling down the column. A mechanical or pneumatic pulse is conventionally applied to the falling aqueous liquid causing the mixed phase to oscillate through the perforations giving suitable organic phase droplet formation and hence suitable mass transfer between the first and second liquids. Chemical species, eg metal ions, dissolved in one liquid are transferred to the other liquid which is thereafter allowed to separate. The application of an electromagnetic field in accordance with the present invention causes the aqueous liquid to oscillate by the force on the liquid thereby to assist the mass transfer process with or without a conventional applied mechanical or pneumatic pulse of energy. The applied field in the method according to the present invention may be provided by known means, eg an electrical coil fed with alternating electrical energy supplied by a suitable high frequency oscillator. The magnitude and frequency of energy supplied to the coil will depend upon the application to which the invention is being put. However, in general, the field is required to be suitable to provide movement of the first liquid relative the second liquid without causing overheating of the liquids, eg causing boiling. Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a schematic flow sheet diagram illustrating a mixer-settler solvent extraction process utilising the assistance of an applied high frequency electromagnetic field. Figure 2 is a cross-sectional side elevation of a pulsed-column solvent extraction process utilising the assistance of an applied high frequency electromagnetic field. In Figure 1, incoming solvent streams are fed into a mixer via inlets 3, 5. Inlet 3 provides a feed of an organic solvent phase and inlet 5 provides a feed of an aqueous phase. The two phases are contacted and stirred together in a mixer 1 and species dissolved in one phase, eg actinides dissolved in the aqueous phase, may thereby be transferred to the other phase. The mixed phases are then fed into a separator 7 in which the phases are allowed to settle again. A high frequency electromagnetic field is applied by a coil 9 around the separator 7 and the field assists the separation of the phases in the separator 9 in the manner described above so that two layers are formed in the separator 7, a lighter one 8 containing the organic phase and a heavier one 10 containing the aqueous phase. The respective phases are thereafter extracted via outlets 11, 13 and treated in a known way. In Figure 2 a pulsed solvent extraction column 15 comprises a top settler 17 and a bottom settler 19 and a body 18 between the two. The body includes a series of perforated plates 20 at stages along its length. Aqueous feed solution is introduced at an inlet 21 in the top settler 17 and an inlet 23 mid-way along the body 19. The feed at inlet 23 contains dissolved species to be removed from the aqueous solution in which they are contained (eg actinides contained in nitric acid solution) . Organic solvent feed is introduced at inlet 25 into the bottom settler 19. In use, the aqueous feed falls through the column 15 passing through the perforations in the plates 20 to form droplets and the aqueous phase contacts with the organic phase moving as a continuous phase up the column 15. Coils 27 provided adjacent selected plates 20 (or all plates 20) each supply a high frequency electromagnetic field to cause pulsing of the aqueous phase droplets in the manner described above to assist contacting of the respective phases. Species dissolved in the aqueous phase are transferred to the organic phase in the column 15 and such phase is extracted as a product at an outlet 29 from the top settler 17. Aqueous phase product (known as raffinate) is extracted at an outlet 31 from the bottom settler 19.

Claims: Claims 1. A method of separating a first liquid from a second liquid in a mixture or suspension of the two liquids, the first liquid having a substantially higher electrical conductivity than the second liquid, which method includes the step of applying a high frequency electromagnetic field through the mixture or suspension. 2. A method as in Claim 1 and wherein the said field a single phase or multi-phase field. 3. A method as in Claim 1 or 2 and wherein the first liquid is a conductor and the second liquid is an insulator. 4. A method as in Claim 3 and wherein the first liquid is water or an aqueous solution and the second liquid is an organic liquid. 5. A method as in any one of the preceding Claims and wherein the step of applying an electromagnetic field is used in addition to one or more other steps to separate the liquids. 6. A method as in any one of the preceding Claims and wherein the applied electromagnetic field has a frequency greater than 5 kHz. 7. A method as in Claim 6 and wherein the applied frequency is from 50 kHz to 500 kHz. 8. A method as in any one of the preceding Claims and wherein the applied field is sufficiently great to generate a force per unit volume greater than the density difference between the first and second liquids but not sufficiently great to boil either of the liquids by the heat generated therein. 9. A method as in any one of the preceding Claims and wherein the method comprises operation of a solvent extraction process in which liquids are contacted to extract chemical species from one liquid to the other and then separated and the said field is applied in the separation stage. 10. A method as in Claim 9 and wherein the said solvent extraction process includes use of a mixer-settler and the applied field is employed in the settler stage. 11. A method as in Claim 9 and wherein the said solvent extraction process includes use of a pulsed column and the applied field is employed to pulse liquid. 12. A method as in any one of Claims 1 to 8 and wherein the method is employed to separate water from aviation spirit. 13. A method as in Claim 1 and substantially the same as any one of the methods described hereinbefore with reference to the accompanying drawings.

Inventors: BAILEY GEOFFREY HORROCKS, BAILEY, GEOFFREY, HORROCKS
Applicants: BAILEY GEOFFREY HORROCKS, BRITISH NUCLEAR FUELS PLC, BAILEY, GEOFFREY, HORROCKS
Application year: 1994
Main classification: B01D17
IPC codes: B01D11, B01D17, B03C1, C10G33